This article is from the Beginner Motorcycle Info FAQ, by firstname.lastname@example.org (Ravi Narayan) with numerous contributions by others.
Get one, get one immediately, and get a full face one, they offer
more protection than half face helmets. Order it early or you will
either have to sit waiting a few days after you get your bike or
risk helmet-less rides. You can get one immediately from a dealer
but you will pay $50-$80 more than if you get it mail order. Order
the best one you can afford, here isn't the place to skimp and
order a $50 one (unless of course you have the infamous $50 head).The
Shoei RF-200 is a good helmet at a reasonable price (about $140
mail order). Phone numbers for mail orders are included at the end.
The fit of the helmet is important, it should be tight enough that
the pads firmly touch your cheeks. It should be tight enough that
you can't grab it and roll it off or rotate it from side to side
very much. But, it should be loose enough that it doesn't pinch or
bind your cheeks or forehead, this will cause fatigue. Sizes vary
by brand, A large in one brand will be like a medium in another
brand. You must try on the exact brand/model in a new (not used)
version before buying. Go to a dealer or cycle shop and try on
several, then order mail-order. (and buy something small from the
dealer for his/her trouble). If it comes in the mail and doesn't
fit right, return it!
Some people claim that helmets restrict your vision, it's not true.
Others claim it reduces your hearing, it actually helps you hear over
the engine. As for the claim they cause neck injuries, I can't offer
any evidence on that but I will take my chances. Wear one for a month
and decide for yourself. The first week it will feel weird (especially
a full face) but it will quickly become as comfy and familiar as your
old fuzzy blanket.